[RFI] looking for larger size ferrites

[Jim Brown]

On 5/4/2020 8:51 AM, Grant Saviers wrote:
> Has anybody characterized these as alternatives to Fair-Rite #31?

A major problem with products from ferrite vendors other than Fair-Rite 
is that their technical data sheets don't come close to providing 
sufficient information to an engineer trying to use them for anything 
other than a single pass through the core. I spent more than a year 
measuring hundreds of samples (that I had to buy) of the two Fair-Rite 
#31 cores that were useful, selecting cores at the limits of their 
tolerances, winding and measuring chokes on each of those limits cores, 
entering data in spreadsheets for each core, and producing winding 
recommendations on the basis of worst case cores for each band.

#31 material is quite unique, in that it is an MnZn material whose 
dimensional resonance and circuit resonance combine to provide high 
bandwidth at HF. It is also rather lossy in that range, which is 
important for chokes.

Most other materials are NiZn, and are far less lossy, which means 
higher Q, which means a narrower resonance, which, combined with very 
broad component tolerances, makes choke designs far less repeatable. 
THIS is why Fair-Rite #43 and #52 are far less useful at HF.

Last year, seeing G3TXQ's recommendations for chokes on #52, I bought 
ten cores each from four different industrial distributors that stocked 
them and repeated the process of selecting limits cores, then winding 
chokes based on his recommendations. I could not repeat his results, NOT 
because of measurement error on his part (his setup was excellent), but 
because of component tolerances.

Others are selling #77 or #78 cores as a solution for HF, but the 
dimensional resonance of these MnZn is in the AM broadcast band, making 
them far less useful at MF or HF. Simply put, they are power products 
for use in low frequency electronics, not for suppression, and the 
Fair-Rite catalog makes that clear.

73, Jim K9YC